Such processing of binary data protected by encryption for smart cards is described in the publication "Federal Information Processing Standards Publication" (FIPS PUB 46), Jan. 15, 1977, National Bureau of Standards of the US Department of Commerce which was last reaffirmed Jan. 22, 1988 as FIPS PUB 46-1. This publication proposes a standard processing for the input data as well as for the secret key, and notably permutations to be executed thereon on the basis of one or more correspondence tables. When such an operation is performed on the data used within a smart card, such execution must be as fast and also as compact as possible, the length of the code permissible for a smart card being very limited. Therefore, the execution of the permutation on the key is costly as regards processing time and code length. For the execution of an irregular permutation, there are customarily two methods. The first method consists in the execution of each permutation on one bit after the other, the second method consisting of reading in the correspondence table the destination address of each bit in succession. The first method is comparatively fast but costly as regards code length, the second method necessitates only a very short program but takes a long time for executing.